1. Field of the Invention
This invention relates to an atmospheric pressure chemical ionization mass spectrometry for analyzing a sample gas separated by gas chromatography.
2. Description of the Related Art
In the description to follow, gas chromatography will be abbreviated as “GC”, a mass spectrometer, as “MS”, an apparatus including the combination of the gas chromatography and the mass spectrometer, as “GC/MS”, an atmospheric pressure chemical ionization source, as “APCI source”, an atmospheric pressure chemical ionization mass spectrometer (apparatus including the combination of APCI and MS), as “APCI/MS”, a chemical ionization source, as “CI source”, an electron impact ion source, as “EI source”, and an apparatus including the combination of the gas chromatography and the atmospheric pressure chemical ionization mass spectrometer, as “GC/APCI/MS”.
GC/MS is a known analytic technology. APCI/MS is an apparatus that ionizes trace components in a mixed sample and detects them with a high level of sensitivity by using an ion-molecule reaction. This apparatus has been utilized for the analysis of the trace components contained in environmental samples and living body samples. A report of a high sensitivity analytical apparatus including the combination of GC and APCI/MS, for analyzing various trace impurities inclusive of semiconductor specific gases (refer to JP-A-9-15207) and a report of an analytical apparatus including the combination of GC and APCI/MS, for analyzing trace impurities in gases for the measurement of the trace impurities of ppb to sub-ppb levels in various high impurity gases (refer to JP-A-295269) have been made. Another report (refer to JP-A-6-310091) is known that separates an ion generation portion and a sample gas introduction portion and efficiently transports ions into apertures so that trace impurities can be measured with a high sensitivity and stably for a long time even for those semiconductor material gases such as monosilane and disilane that are likely to invite contamination inside an ion source.
According to JP-A-9-15207, the sample gas separated in a column of the GC is mixed with a carrier gas and is then introduced to an APCI source through a line. The flow rate of the carrier gas flowing through the line is set to a minimum flow rate that can keep the atmospheric pressure inside a discharge space of the APCI source for the purpose of high sensitivity analysis. An orifice is disposed so that the discharge space can be kept steadily and easily at the atmospheric pressure and the flow rate of an exhaust line can be regulated to the minimum flow rate.
According to JP-A-11-295269, there is an introduction system extending to APCI/MS through GC and a makeup flow passage connected to an outflow passage from GC is disposed. The reference puts the following description:
“The flow rate of the sample gas introduced into APCI/MS is generally from hundreds of mL (milliliters)/min to several L/min. The flow rate of the carrier gas of GC is generally 20 to 50 mL/min. It is therefore necessary to add a corresponding amount of a makeup gas from the makeup passage to the outflow gas from GC. Because this makeup gas dilutes the impurities as the assumed objects, however, its flow rate must be reduced as much as possible. To improve the sensitivity of the impurities in the analysis through GC, therefore, it is preferred to minimize the flow rate of the sample gas introduced into APCI/MS such as about 300 mL/min.”
JP-A-6-310091 puts the following description:
“. . . includes an ion generation portion as an area for generating a primary ion, having an introduction port of a primary ion generation gas and ionization means, a sample introduction portion as an area into which the sample gas is introduced, a mixing portion in which the primary ion supplied with the primary ion generation gas from the ion generation portion is mixed with the sample gas and ions of an object material in the sample gas are generated by an ion-molecule reaction, and a mass analysis portion which is held under high vacuum and in which the ion introduced from the mixing portion through an aperture is subjected to mass separation, wherein at least one of means for transporting the ion to the aperture by adding an electric field by an electrode disposed in the mixing portion, means for constituting the ion generation portion and the sample introduction portion into a double wall tube and making the flow of the primary ion generation gas and the flow of the sample gas parallel to each other in the mixing portion, and means for discharging the gas reaching the aperture in a direction extending along a surface having the aperture is added so as to increase a detection ion amount”.
In the column of “Prior Art”, JP-A-6-310091 reads as follows:
“The primary ion is injected with the primary ion generation gas under the atmospheric pressure and the primary ion is mixed with the sample gas at the atmospheric pressure ahead of the aperture of the mass analysis portion. The ion of the object material is generated by the ion-molecule reaction. Because the ion generation portion and the sample introduction portion are separated in APCI/MS, the ion generation portion is not contaminated even when a gas flowing out from GC and LC (liquid chromatograph) is used as the sample gas and ionization can be carried out stably for a long time”.
Ionization of the sample gas by chemical ionization in air is well known (refer to “The Journal of Chemical Physics”, Vol. 52, 212–221(1970)).
In JP-A-9-15207, the sample gas separated in the column of GC is mixed with the carrier gas and is introduced into the APCI source through the line. In JP-A-11-295269, the outflow gas from GC is introduced into APCI/Ms after the makeup gas is added from the makeup passage. Therefore, the outflow gas from GC is diluted. Because ionization of the sample described in “Prior Art” in JP-A-6-310091 is conducted at the atmospheric pressure, the outflow gas from GC is diluted, too. Even when the outflow gas from GC is allowed to flow through the sample introduction portion in the construction for increasing the detection ion amount in JP-A-6-310091, the outflow gas is diluted by the primary ion generation gas in the mixing portion. As described above, the prior art technologies do not sufficiently take dilution into consideration when the outflow gas from GC is introduced into the APCI source, and are not free from the problem of the drop of the sensitivity.
The analysis by GC/MS is suitable for separation analysis of a plurality of components contained in a mixed sample, particularly for separation analysis of components having high volatility. The ion source used for GC/MS generally includes a CI source and an EI source. These ion sources are used for ionization under vacuum of about 0.1 torr to several torr. The sample gas separated by GC is introduced into the ion source under this vacuum. However, when the sample contains a large number of impurities, the impurities are adsorbed to the wall of the ion source and to constituent components and remain inside the ion source. Consequently, the ion source is contaminated and an S/N ratio gets decreased. Therefore, maintenance such as frequent cleaning of the ion source is necessary and hinders efficient analysis.
When the APCI source is used for the ion source, on the other hand, ionization of the sample is conducted at the atmospheric pressure and a differential exhaust portion for transporting the ion from the ion source to the mass analysis portion under vacuum is disposed. The ion from the ion source is introduced into the vacuum portion through an ion introduction aperture having a diameter of about 0.1 to about 0.5 mmφ. To keep stable duration of corona discharge, a gas (primary ion generation gas (discharge gas)) of a flow rate of about 0.1 to about 1 L/min must be caused to flow steadily to the ion source and the gas flow rate flowing through the ion introduction aperture is about 0.1 to about 1 L/min.
Because the flow rate from GC is several to dozens of mL/min, the gas from GC is diluted by the primary ion generation gas that is caused to flow to the ion source. However, the problem of the drop of sensitivity occurs when the gas from GC is merely mixed with the primary ion generation gas.